2.1. Prevention of Coccidiosis
Coccidiosis is a costly disease of poultry caused by intracellular protozoan parasites of the genus Eimeria. The disease is endemic in the large, intensive poultry breeding establishments in this country, and the estimated cost of control of the disease through chemotherapy exceeds $100 million each year. Resistance to the anti-coccidial drugs develops, necessitating a continuing development of new agents, at a time when drug development is becoming increasingly expensive and consumer acceptance of drug residues in food animals is diminishing.
Protective immunity to natural coccidiosis infection has been well documented. Controlled, daily administration of small numbers of viable oocysts for several weeks has been shown to result in complete immunity to a challenge infection of a normally virulent dose [Rose et al., Parasitology 73:25 (1976); Rose et al., Parasitology 88:199 (1984)]. The demonstration of acquired resistance to infection suggests the possibility of constructing a vaccine to induce immunity in young chickens, circumventing the need for chemical coccidiostats. In fact, such a concept has been tested in the Coccivac.RTM. formulation of Sterwin Laboratories. Opelika, Ala.
With a view to producing a coccidiosis vaccine, Murray et al., European Patent Publication No. 167,443, prepared extracts from sporozoites or sporulated oocysts of Eimeria tenella which contain at least 15 polypeptides, many of which were associated with the surface of the sporozoite. Injection of these extracts into chickens reduced cecal lesions following oral inoculation with virulent E. tenella sporulated oocysts. More recently, Schenkel et al., U.S. Pat. No. 4,650,676, disclosed the production of monoclonal antibodies against E. tenella merozoites. Using these antibodies, Schenkel et al. identified a number of antigens against which the antibodies were directed. By pre-incubating E. tenella sporozoites with these antibodies and then introducing the treated sporozoites into the ceca of chickens, Schenkel et al. were able to show some reduction in cecal lesion scores, compared to untreated sporozoite controls.